WASHINGTON, Sept. 19 - In 1953, the chief executives of the country's leading cigarette companies and officials from a major public relations firm gathered at the Plaza Hotel in Midtown Manhattan.

What they discussed that day and what happened over the next 50 years as a result go to the heart of the biggest legal challenge the tobacco industry has ever faced.

In a nonjury trial scheduled to start here on Tuesday in Federal District Court, the government is seeking to strip the companies - disgorgement, in legal terms - of $280 billion that Justice Department lawyers say was earned through fraud. As the largest civil case ever prosecuted under the federal Racketeer Influenced and Corruption Organizations Act, it has the potential to put the companies out of business.

Five years in preparation, at a cost to the government of $135 million, the trial is scheduled to last at least six months, with 100 witnesses expected to testify in person and 200 others through depositions or testimony in other trials.

Company lawyers say the hotel meeting produced only a research organization, now defunct, to study smoking and health. The government's lawyers say the meeting led to a widespread conspiracy of deception that remains in effect, reflecting a carefully built strategy to misrepresent the addictive nature of cigarettes, lie about the health risks of secondhand smoke and direct marketing efforts at young people to sustain a large population of smokers.

"The government has provided extensive evidence to support our case," Peter D. Keisler Jr., assistant attorney general for the civil division of the Justice Department, said in a statement. "We look forward to presenting it in court."

The defendant companies - Philip Morris USA; its parent, the Altria Group; the R.J. Reynolds Tobacco Company; the Brown & Williamson Tobacco Corporation, which merged over the summer with Reynolds; the Lorillard Tobacco Company, a subsidiary of the Loews Corporation; British American Tobacco; and the Liggett Group - say that the government's case is groundless.

They deny engaging in a conspiracy and accuse the government of distorting history to drive them into bankruptcy. They also say that under the terms of a 1998 settlement with 46 states that sued to recover nearly $250 billion for the health care costs of smoking, the companies have already complied with orders that the government is seeking in the lawsuit, like public disclosure of company research relating to smoking and bans on marketing to children.

William S. Ohlemeyer, vice president and associate general counsel for Altria, said the judge, Gladys Kessler, could decide for the government only if it could show that a pattern of fraud in the past was evidence of fraud in the present and future.

Mr. Ohlemeyer said that past behavior was debatable. He said positions the companies once held - that smoking does not cause disease, for example - "can be wrong without being evidence of committing fraud."

As for the present and future, he said, the 1998 settlement created so much government oversight that continuing fraud would be impossible. "The court is required to review the totality of circumstances," Mr. Ohlemeyer said in a conference call with reporters last week. "It's difficult for the government to argue that the past is a reasonable predictor of the future. It ignores a detailed list of how cigarettes are sold today versus the past."

Filed in 1999, the case originally included charges to recover federal health care costs due to smoking. Judge Kessler dismissed them, leaving two counts under the racketeering act. The Justice Department has aggressively pursued those charges despite several efforts by Congress to block financing for the case.

"With President Bush's election, the tobacco industry thought they were going to have a friend who would get the lawsuit dismissed," said William V. Corr, executive director of the Campaign for Tobacco-Free Kids. "Fortunately none of those efforts succeeded, and when terrorism became such a prominent issue for our government and the public, it appears that the effort to undermine the lawsuit diminished."

Full victory for the government after appeals would have major financial consequences for the companies. Martin Feldman, an analyst for Merrill Lynch who tracks the tobacco industry, estimated that the combined net worth of the companies, which account for 85 percent of the domestic cigarette market, was less than $200 billion, at least $80 billion less than what the government is seeking.

But whether the government has the right to seek disgorgement under its theory of the case is now before the United States Court of Appeals for the District of Columbia Circuit. That court has agreed to hear an appeal of Judge Kessler's decision in May dismissing the companies' request that the disgorgement claim be thrown out. Oral arguments are scheduled for Nov. 17.

If Judge Kessler's ruling is overturned, Mr. Feldman said, "this case ceases to be newsworthy."

Justice Department officials, who discussed the case in a background briefing with reporters under the promise that their names not be used, said the government would try to show a conspiracy to sell cigarettes through intentional misstatements about smoking and health, the addictive nature of cigarettes, the manipulation of nicotine as the addictive ingredient, the marketing of low-tar cigarettes as safer and the suppression of evidence that would adversely affect sales.

The companies' chief strategy is to direct the judge's attention to industry reforms since 1998. "The focus of this case should be on recent history, the activities of the defendants today, and an actual threat of a specific ongoing or future violation," the companies said in court documents.

"We intend to rebut the charges that fraud was committed in the past," Mr. Ohlemeyer said. "And we're going to make it very clear to the judge that no evidence currently exists of an intent to commit fraud in the future."

To speed the case along, Judge Kessler has instructed each side to conduct direct examinations of witnesses outside court before their appearances. That means that when David A. Kessler, a former commissioner of the Food and Drug Administration who is not related to the judge, takes the stand on Thursday as the first witness, company lawyers will immediately cross-examine him about testimony that was filed last week.

In that testimony, Dr. Kessler recounted efforts by the agency during his tenure, 1990 to 1997, to regulate nicotine as a drug in the belief that the cigarette companies manipulated the level of nicotine to sustain addiction.

Citing company documents, he told the court that the companies had known for decades that nicotine was a drug but that the agency's effort at regulation had been challenged by a lawsuit from the industry that reached the Supreme Court.

In 2000, the court ruled 5 to 4 for the industry, saying that Congress did not intend the agency to regulate cigarettes. But when Dr. Kessler was asked if any justice took issue with the agency's findings, he said, "No."